The present invention relates to a process for the transmission in blocks of digital information words. It more particularly relates to a coding and decoding procedure in which transmission blocks are formed, each being constituted by a sequence of associated digital information words and redundancy words, the latter being able to detect and correct, on reception, long error bursts, such as those encountered in digital recording on magnetic tape.
In conventional manner, in the field of digital recording on magnetic tape, the information to be recorded is organized into frames, each consisting of a synchronization word, an identification word, one or more information words and an error detecting code, which may not detect all the errors.
In order to improve the error detecting and correcting capacities, it is known to regroup the frames into sets of frames and link the homologous informations (i.e. bits of the same order) of the frames of the same set by simple mathematical relations, such as the parity of the sum of the homologous bits. It is also known to form a transmission block by interlacing the sequences of frames of several linked sets, so as to make the probabilities of errors on the frames of the same set independent of one another.
The article Improved two channel PCM tape recorder for professional use by K. Tanaka et al presented at the 64th congress of the Association of Audio Engineers in November 1979 in New York describes such a block transmission process for digital information words. Recording takes place on an 8 track magnetic tape, 6 of these tracks being allocated to information words and two to redundancy words linking the homologous informations of the frames of the same set. These redundancy words are obtained with the aid of a code of the Reed-Solomon type.
This known process is not completely satisfactory because the Reed-solomon code is complex. The detection and correction of errors by means of this code consequently require long and complicated calculations. Moreover, the correction capacity of said process, i.e. the probability of having a non-correctable error is proportional to p.sup.3 when the probability of having an error is equal to p. This correction capacity is not very high. The performance to complexity ratio is consequently mediocre.
Transmission processes are known in which the probability of a non-correctable error is proportional to p.sup.4. Such a process is described in document FR-A-2 467510.
In this process, the information words are subdivided into a sequence of even information words and a sequence of uneven information words. These sequences are coded and transmitted with a time lag. This method makes it possible to ensure that a fault on the transmission channel or recording medium does not lead to an error on two consecutive information words. An information word recognized as being erroneous can consequently be corrected or, if the correction is not possible, interpolated as from the preceding information word and the following information word. This interpolation is obviously only useful if the successive information words are correlated, which is particularly the case in digital recording of a sound signal.
In this process, the transmission or recording format has a sliding structure, i.e. the interlacing of the frames is such that it is impossible to define independent blocks, as was the case in the aforementioned article. This leads to complications during the electronic setting up of sound programs, particularly due to the fact that the sliding structures are broken at the installation point, which leads to a correction capacity loss.